It is well known that optical devices, such as modulators, directional couplers, polarizers, etc., can be integrated into a common layer of optical material, referred to here as a substrate. A suitable layer of optical material typically has a transmission loss of less than 10 db per centimeter (db/cm) at a desired optical wavelength. More typically, the loss is less than a few tenths of a db/cm. The material is usually chosen to have electro-optic properties when integrating electro-optical devices such as modulators. For example, lithium niobate (LiNbO3) is widely used for this purpose. However, where electro-optical devices are not necessary, other materials, including glass, may be used. Each type of optical device employs integrated optical waveguides for both fabrication and interconnection of the optical devices on a common substrate. To form optical waveguides in the substate, a common technique involves selectively diffusing titanium into the substrate where a waveguide is desired. These waveguides typically have very low transmission loss (typically tenths of a db per cm.) along straight sections of waveguide. However, should the direction of the waveguide change (a bend), the loss increases significantly in relation with the radius of curvature of the bend. Such losses are discussed in "Improved Relations Describing Directional Control in Electromagnetic Wave Guidance", by Marcatili and Miller, Bell System Technical Journal, Vol. 48, No. 7 (Sept. 1969), pp. 2161-2188. In summary, the losses are caused by two mechanisms: radiation loss caused by the energy distribution (Gaussian) of the light in the waveguide, being non-zero outside of the waveguide, into a region capable of supporting a radiating wave, and mode conversion loss by the light coupling to lossy higher order modes as it changes direction. Such losses limit the radius of curvature of a bend in an integrated optical waveguide for a predetermined amount of loss, resulting in optical devices and interconnection of which are physically large on the substrate. This places a limit on the number of such devices that can be constructed on a given substrate size. One technique which may be used in reducing bend loss is disclosed in an article titled "Dielectric Optical Waveguide Tilts With Reduced Losses", by E. G. Neumann, published in the proceedings of the Seventh European Conference on Optical Communication, Sept. 8-11, 1981, pp. 9.3-1 to 9.3-4. The technique involves modifying the refractive index of the substrate surrounding the curve (tilt) of a waveguide therein to refract the energy propagating in the waveguide around the turn. As shown in FIG. 1d, the index of refraction of the substrate in contact with the outside of a turn of the waveguide is modified to be lower than that of the substrate to speed up the propagation of energy outside of the waveguide. Another technique for reducing bend loss is disclosed in "Greatly Reduced Losses for Small-Radius Bends in Ti:LiNbO3 Waveguide", by S. K. Korotky, et al, published in the proceedings for the Third European Conference on Integrated Optics, May 6-8, 1985, pp. 207-209. This technique, referred to as CROWNING and applied to curved waveguides formed in a substrate, varies the refractive index inside the waveguide by a plurality of dielectric prisms so as to locally influence the direction of light propagation. However, an extra diffusion step is required to form the prisms, scattering loss is increased by the introduction of features (prisms) within the waveguide and this technique is wavelength dependent.
To be practical, the optical devices on the common substrate must couple to the "outside" world. A common technique uses optical fibers to couple optical energy to and from the integrated optical devices. For example, an optical fiber couples light from an external laser to the substrate for modulation by a modulator formed in the substrate and a second optical fiber couples the so modulated light to a distant optical receiver. For integrated optical waveguides, of which the exemplary modulator is constructed, the mode of propagating optical energy is typically oval in shape. However, the mode of optical energy propagating in the fiber light-guide is typically circular. Because the modes in each type of waveguide (integrated optical waveguide and optical fiber) are not substantially the same at the interface between the two waveguides, full optical energy transfer between the waveguides does not occur. The inefficiency of optical energy transfer between the optical fiber and the integrated optical waveguide due to the mode mismatch is included in the coupling loss.